In a gas and steam turbine installation, the heat contained in the expanded flue gas of the gas turbine is used to generate steam for the steam turbine. Heat transfer takes place in a waste-heat steam generator (WHSG), which is connected downstream of the gas turbine and in which heating surfaces in the form of heating tubes or tubes are arranged in tube bundles and connected to the water-steam circuit of the steam turbine.
The waste-heat steam generator and the steam turbine form the steam component of the gas and steam turbine installation. In general, the gas turbine is designed in such a way that the exhaust gas parameters thereof (temperature, mass flow, excess pressure) match the inlet parameters of the downstream systems within wide operating ranges.
During the operation of an installation of this kind, however, the heat introduced into the steam generator differs in different operating states. In this case, critical operating states often occur owing to the different dynamic behavior of the installation components. These arise from the relatively large delay or response times and different sensitivities of the steam turbine installation compared to the gas turbine installation. Thus, in start-up or load-change mode, for example, especially in the case of “quick starts” or rapid load changes, large temperature gradients and/or changes in temperature with respect to time and significant changes in the mass flow of the expanded flue gas occur. In start-up and load-change mode, the gas turbine installation is therefore generally restricted in terms of temperature and power.
A temperature measuring device and control system for the hot gas temperature of a gas turbine is described in EP 1 462 633. EP 1 442 203 B1 describes a method for controlling the cooling air mass flows of a gas turbine group. However, the problem of the occurrence of large temperature gradients in the case of quick starts or rapid load changes is not countered in this document.
In order to counteract this problem, the gas turbine and the steam component can be decoupled by means of a bypass stack connected between the gas turbine and the waste-heat steam generator. In the event of a case of operation which represents a critical state for the steam generator, the expanded flue gas can thus be discharged via the bypass stack ahead of the steam generator. For a gas and steam turbine installation, this involves a reduction in power and reduced efficiency.
If there is no bypass stack, the gas turbine and the waste-heat generator cannot be decoupled from one another. In the case of a gas and steam turbine installation without a bypass stack, a critical operating state occurs especially if the installation is operated in the part-load range, particularly at high ambient temperatures. Since there is no bypass stack, the steam generator is unavoidably operated at the same time. To ensure that the temperature in the steam generator remains below the permitted design limit for the steam lines, the steam turbine and/or the bypass station, the steam produced in the steam generator must be cooled. Consequently, the management system of the power plant must ensure that an impermissible increase in the temperature of the material of the heat exchanger tubes and headers due to impermissibly high temperatures at the heating surfaces is avoided in the case of the unavoidably falling steam production in the steam generator. This requires the artificial lowering of the flue gas temperature of the gas turbine. Such a mode of operation is associated with losses in the efficiency of the overall installation since the heat energy in the flue gas is reduced and thus remains partially unused.
Owing to the close coupling and dependence between the gas turbine and the steam component, this unavoidable operation has generally negative effects on the flexibility of the installation since the steam component responds to the waste heat provided by the gas turbine only with a significant delay and in a very complex manner.
To counter this problem, European Patent EP 0 579 061 A1 describes a device and a method for operating a gas and steam turbine installation which allows as gentle as possible operation with, at the same time, a high overall efficiency in all operating states, that is to say also in the case of critical states in the start-up or load-change mode. For this purpose, it discloses a method in which, when a critical operating state of the gas and steam turbine installation is reached, the temperature of the expanded flue gas is reduced by introducing water into the expanded flue gas. In this process, water is sprayed into the flue gas duct by means of a spraying device. The amount of water to be introduced is determined in dependence on the flue gas temperature. For this purpose, an actuator, a control element and two temperature sensors are provided in the flue gas duct. The temperature sensors are arranged after the injection device in the flue gas duct. The temperature sensors thus detect the temperature of the flue gas which has already been cooled by injected water. The control element compares the measured actual temperature with a predetermined setpoint temperature and adjusts the amount of water to be introduced if there is a difference between the setpoint and the actual temperature.
The disadvantage of the prior art is, in particular, the fact that the measurement results are falsified by the spraying in of water ahead of the measuring points, which leads to incorrect determination of the combustion temperature in the gas turbine and thus to a highly negative intervention in the control thereof in the case of steady-state operating modes but especially also in the case of non-steady operating modes of the gas turbine. The highly distorted measured values occur because major fluctuations in temperature distribution are caused by the water sprayed in.